The present invention relates generally to sealing apparatus for a ventilation system for an electrical generator and particularly relates to sealing apparatus for generator cooling fans using non-metallic brush seals at fan blade tips for eliminating or minimizing leakage flows and enhancing fan efficiency.
During normal operations of electrical generators, heat is generated in conducting coils, rotor and stator cores, frame, etc., due to the copper, iron, windage and load losses. To ensure reliable operation of the generator, such produced heat must be dissipated efficiently from the generator by a ventilation system. Ventilating fans mounted at the rotor shaft ends play a critical role in the ventilation system by driving cold gas to cool the stator and rotor windings and cores and thus assure operating temperatures of the generator below allowable limits.
In a typical generator, with a ventilating fan mounted on an end of the rotor, cooling gas flows essentially into four branches of the generator: the rotor stator gap, the rotor sub-slot, outside space block and ventilating holes to a heat exchanger. As the cooling gas leaves the fan and approaches the rotor, the rotating surfaces of the end region of the rotor act to accelerate the gas to rotor speed. This facilitates pumping of the cooling gas passing through the armature bars. The cooling flow splits at the front of the centering ring to enter the rotor sub-slot and rotor stator gap, respectively. Then all of the cooling gas passes through the stator core in a radially outward direction. Because all of the hot gas of the rotor must pass through the stator core, the distribution of gas through both rotor and stator are interrelated and will affect the stator operating temperatures. To best cool the ends of the armature bars near the series loop caps, the gap between the edges of armature bars and the tip of a gas shield is used to adjust the flow rate.
The end winding region is located at the two ends of the generator. A large number of parts and components are in the region, including a ventilating fan, a gas shield, armature bars, a retaining ring, a centering ring, rotor end windings, power connection rings and a stator flame. As a result, the flow field in this region is considerably complex. With large-scale and high-powered generators, the cooling flow at the end winding region is particularly important because it may result in local hot spots in the end winding bars and a large ventilating windage loss. How well the armature winding of the generator is cooled has a significant influence on the overall size of a synchronous generator.
The design of a generator fan is based on a required pressure at a specified flow. The fan must run at rotor shaft speed and the dimensions are limited by the geometry of the end structure of the generator. The fan efficiency is directly dependent upon the ability of the seals to prevent leakage. Generators typically employ rigid, triangularly-shaped teeth, i.e., labyrinth-type seals, to control leakage between the rotating fan blades and the stationary gas shield. A radial clearance is maintained between the fan blade tips and the labyrinth teeth of the gas shield to avoid damage to the fan blades. However, this clearance results in leakage flows and thus a lowering of the efficiency of the fan. Accordingly, there is a need for an enhanced seal to facilitate reduction in leakage flows between the fan tips and generator housing in a cost-effective and reliable manner.
In accordance with a preferred embodiment of the present invention, there is provided a combined brush seal such as double-row, bi-directional or slanted brush seals, and a conventional labyrinth seal which eliminates or minimizes gas leakage flows through the gap between the fan blades and the gas shield. The brush seals are formed of non-metallic materials such as plastic fibers and/or Kevlar(trademark). Non-metallic brush seals afford low bristle stiffness thereby facilitating a zero radial clearance between the fan blade tips and the brush bristles. Also, with low stiffness the cant angle is no longer a controlled design parameter since the cant angle is established naturally by the blade rotation. Further, the low stiffness and non-metallic bristles accommodate radial shaft excursions without damage to the shaft or seal.
In a preferred embodiment, two double-row bi-directional brush seals are mounted on a brush seal holder on opposite sides of labyrinth seal teeth. The brush seal holder is secured to the generator housing, particularly the gas shield, to form an annular seal about the tips of the fan blade at the end of the rotor. With the labyrinth teeth between the brush seals, vortex flows introduced from the upstream brush seal do not influence significantly the downstream brush seal. To further enhance sealing, notches may be formed on the tips of the fan blades to introduce a small radial step on each side of the blade. Thus, the tips of the bristle packs lie at different radial positions, affording increased sealing effectiveness. Slanted brush seals may also be utilized adjacent the sides of the fan blades. With the bristles inclined toward opposite sides of the fan, any leakage flow requires essentially two 90xc2x0 turns in opposite directions which substantially improves sealing effectiveness. Also, the bristles of the slanted brush seals may be slanted upstream toward the leakage flow to increase resistance to the leakage flow. The labyrinth teeth may likewise be slanted toward the leakage flow.
In a preferred embodiment according to the present invention, there is provided sealing apparatus comprising a rotary component mounted for rotation about an axis, a stationary component surrounding the rotary component, a seal between outer diameter margins of the rotary component and the surrounding stationary component, the seal including an annular labyrinth seal carried by the stationary component and having a plurality of teeth spaced from outer margins of the rotary component and an annular brush seal on each of axially opposite sides of the labyrinth seal teeth, the brush seals having bristles extending toward the margins of the rotary component and terminating in tips adjacent the margins whereby leakage through any gap between the margins of the rotary component and the stationary component are minimized or eliminated.
In a further preferred embodiment according to the present invention, there is provided sealing apparatus for a ventilation system in an electrical generator having a rotor, a stator and a housing for the rotor and stator, comprising a fan for mounting on an end portion of the rotor with portions of the generator housing surrounding the fan, a seal between outer diameter margins of the fan and the surrounding housing portions, the seal including an annular labyrinth seal carried by the surrounding housing portion and having a plurality of teeth spaced from outer margins of the fan and an annular brush seal on each of the opposite sides of the labyrinth seal teeth, the brush seals having bristles extending toward the margins of the fan and terminating in tips adjacent the fan margins whereby leakage through any gap between the margins of the fan and the housing portion are minimized or eliminated.